Azobenzene undergoes trans → cisisomerization when irradiated with light tuned to an appropriate wavelength. The reverse cis →transisomerization can be driven by light or occurs thermally in the dark. Azobenzene's photochromatic properties make it an ideal component of numerous molecular devices and functional materials. Despite the abundance of application-driven research, azobenzene photochemistry and the isomerization mechanism remain topics of investigation. Additional substituents on the azobenzene ring system change the spectroscopic properties and isomerization mechanism. This critical review details the studies completed to date on the 3 main classes of azobenzene derivatives. Understanding the differences in photochemistry, which originate from substitution, is imperative in exploiting azobenzene in the desired applications.

Photoisomerization in different classes of azobenzene

Reaction dynamics of the photochromic systems such as spiropyran, flugide and diarylethene derivatives in femto- to nano-second time region was reviewed. The mechanisms of rapid reaction channels in various phases and future prospects for photochromic systems were discussed.

Ultrafast Dynamics of Photochromic Systems.

In this paper, we identify the most efficient decay and isomerization route of the S1, T1, and S0 states of azobenzene. By use of quantum chemical methods, we have searched for the transition states (TS) on the S1 potential energy surface and for the S0/S1 conical intersections (CIs) that are closer to the minimum energy path on the S1. We found only one TS, at 60° of CNNC torsion from the E isomer, which requires an activation energy of only 2 kcal/mol. The lowest energy CIs, lying also 2 kcal/mol above the S1 minimum, were found on the torsion pathway for CNNC angles in the range 95−90°. The lowest CI along the inversion path was found ca. 25 kcal/mol higher than the S1 minimum and was characterized by a highly asymmetric molecular structure with one NNC angle of 174°. These results indicate that the S1 state decay involves mainly the torsion route and that the inversion mechanism may play a role only if the molecule is excited with an excess energy of at least 25 kcal/mol with respect to the S1 minimum...

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On the Mechanism of the cis−trans Isomerization in the Lowest Electronic States of Azobenzene: S0, S1, and T1

The excited-state dynamics of trans-azobenzene were investigated by femtosecond time-resolved photoelectron spectroscopy and ab initio molecular dynamics. Two near-degenerate ππ* excited states, S2 and S3,4, were identified in a region hitherto associated with only one excited state. These results help to explain contradictory reports about the photoisomerization mechanism and the wavelength dependence of the quantum yield. A new model for the isomerization mechanism is proposed.

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Mechanism and dynamics of azobenzene photoisomerization.

Ultrafast photochemical reactions in liquids occur on similar or shorter time scales compared to the equilibration of the optically populated excited state. This equilibration involves the relaxation of intramolecular and/or solvent modes. As a consequence, the reaction dynamics are no longer exponential, cannot be quantified by rate constants, and may depend on the excitation wavelength contrary to slower photochemical processes occurring from equilibrated excited states. Such ultrafast photoinduced reactions do no longer obey the Kasha–Vavilov rule. Nonequilibrium effects are also observed in diffusion-controlled intermolecular processes directly after photoexcitation, and their proper description gives access to the intrinsic reaction dynamics that are normally hidden by diffusion. Here we discuss these topics in relation to ultrafast organic photochemical reactions in homogeneous liquids. Discussed reactions include intra- and intermolecular electron- and proton-transfer processes, as well as photochr...

F.V.R. Neuwahl

Papers

Femtosecond time-resolved UV-visible absorption spectroscopy of trans-azobenzene: dependence on excitation wavelength